Steel bars, one type of ferrous metal product, is produced by rolling. At the time of rolling, however, flaws sometimes occur on the surface of the steel bars. Therefore, inspection for the presence of flaws becomes important in the quality inspection process.
To inspect for flaws, frequently the eddy current flaw detection method is used.
FIG. 1 is a perspective view of a flaw detection device for a steel bar used in the past. The device is comprised of a pre-treatment unit 11, a detection unit 12, and a post-treatment unit 13. The steel bar 11 moves from the pre-treatment unit 11 toward the post-treatment unit 13.
The pre-treatment unit 11 and the post-treatment unit 13 have the same configuration and are comprised of sleeves 111 and 131 comprised of tubular magnetic members and direct current coils 112 and 132 wound around their outsides.
These direct current coils 112 and 132 function to magnetically saturate the ferromagnetic steel bar at a temperature lower than a temperature corresponding to the Curie point by a direct current magnetic flux generated by the direct current coils 112 and 132 so as to raise the flaw detection sensitivity when using the substantial alternating current permeability as the zero point.
The detection unit 12 is comprised of a cylindrically shaped direct current coil 121 and two short tubular detection coils 122 and 123 arranged inside it. The two detection coils 122 and 123 are differentially connected in series and for example are connected to a voltmeter as a measuring device (not shown). The alternating current coil 121 functions to generate an eddy current at the surface of the steel bar, while the detection coils 122 and 123 detect disturbances in the magnetic field due to disturbances in the eddy current caused by flaws on the surface of the steel bar.
With the conventional eddy current flaw detection device, however, maintenance of the flaw detection accuracy required that an approximately room temperature steel bar be moved at a speed of about 2 m/sec, so the productivity ended up low. Due to this:                (1) Since a steel bar is about 800° C. in the withdrawal step, it is necessary to provide a cooling time for cooling them to approximately room temperature before entering the quality inspection process.        (2) Since the speed of movement of the steel bar is a slow one of about 2 m/sec, the inspection itself takes time.        
If trying to inspect a high temperature steel bar at a high speed by an eddy current flaw detection device used in the past, the following problems arise:                (1) Since the pre-treatment unit 11 and post-treatment unit 13 are separated from each other and the sleeves 112 and 132 for preventing contact of the steel bar with the direct current coils 112 and 132 are made of magnetic materials, if increasing the inspection speed to about 120 m/sec, noise unavoidably occurs due to the changes in axial direction of the magnetic field strength.        
FIG. 2 is a view of the distribution of the magnetic field strength in the axial direction of a flaw detection device for a steel bar used in the past. Inside the sleeves 111 and 131, the strength is constant, but at the two ends of the sleeves 111 and 131, the strength sharply changes.
Therefore, when the steel bar passes the two ends of the sleeves 111 and 131 where the magnetic strength changes sharply, current is induced at the surface of the steel bar. If a low speed of about 2 m/sec, the current caused at the detection coils 122 and 123 due to the induction current does not have any effect on the flaw detection, but at a high speed of about 120 m/sec, the current caused at the detection coils 122 and 123 becomes noise and influences the detection accuracy.
Therefore, to make the detection accuracy high, it is important to flatten the distribution of the magnetic strength in the axial direction of the eddy current flaw detection device.
(2) Near room temperature, the relative permeability of a steel bar is substantially constant, so even if the temperature fluctuates somewhat, it is possible to maintain the inspection accuracy.
At 700 to 400° C., however, with a magnetic field able to magnetically saturate a steel bar at room temperature, the relative permeability fluctuates dramatically in accordance with changes in temperature and it is not possible to maintain the inspection accuracy.
Therefore, it becomes important to increase the strength of the direct current magnetic field for magnetically saturating the steel bar and improving the temperature characteristic of the relative permeability.
The present invention was made in consideration of the above problems and has as its object the provision of a flaw detection device for a steel bar enabling inspection of a high temperature steel bar at a high speed.